事故应急救援预案的特点有哪些？
参考答案：①科学性；②实用性；③权威性。
I don't have enough money. Maybe I get a part-time job.[ ]
A. should
B. shouldn't
C. shall
D. had to
旁路式旋风除尘器的除尘效率较高，一般可达到（）。
A.40至50
B.55至60
C.70至80
D.80至90
